Hello. You can't carry out A/B extraction for free base. It is possible to do only with amphetamine salts. You can read manual from main page of Amphetamines section
Amphetamine purification by acid-base extraction
Yes, you have to make solution like a sulfuric acid with acetone.
To cound how much acid do you need we will take
10 ml of amph free base as in example. 10ml of amph base / 0.936 g/ml = 10.6838 g. You have to count amount of moles: 10.6838g /135.21 g/mole = 0.079 moles of amphetamine. To produce monoamphetamine phosphate you have to take same amount of acid as amphetamine.
Hence: 0.079 moles * 97.994 g/mole = 7.743g of pure H3PO4. In case you have 70% acid solution, you have to take (7.743g * 100) / 70 =
11.06 ml of 70% H3PO4.